1. Field of the Invention
The general field of the invention is that of anti-collision systems for aircraft and more particularly that of the presentation of anti-collision information.
2. Description of the Prior Art
For an aircraft in flight, it is vital to know very accurately the aircraft that are situated in its immediate environment in order to avoid any risk of collision. This problem is particularly crucial in a certain number of applications where the aircraft are required to fly at low altitude with possibly reduced visibility conditions. Historically, since the years 1960-1970, a solution that is independent of Air Traffic Control has gradually emerged. This solution is known by the acronym TCAS, standing for “Traffic alert and Collision Avoidance System”.
Today, a number of TCAS families have been developed and are in use:                The first generation, called TCAS I supplies only “Traffic Advisory”, or TA, type alerts to the proximity of intruders, craft or aircraft presenting a risk to the aircraft. TCAS I is primarily used in general aviation, that is in the light aircraft domain.        TCAS II supplies on the one hand TA-type alerts to the proximity of intruders and on the other hand conflict resolution by suggesting avoidance manoeuvres to the pilot. The operating mode is called RA, standing for “Resolution Advisory”. These avoidance manoeuvres are performed in a vertical plane by a climb or a descent of the craft. It is primarily used in commercial aviation. TCAS II was made mandatory in the 1990's on all aeroplanes.        TCAS III, still in development, is an improvement on TCAS II enabling in addition a resolution of the conflicts in RA mode in the horizontal plane by left or right turn manoeuvres.        
The TCAS information can be presented to the pilot in different ways. As an example, FIG. 1 shows the presentation of intruders on a Navigation Display (ND) type screen. The intruders are presented in a 2D horizontal plane relative to the aircraft 100 in so-called “ROSE” mode, alluding to the French word for compass. The aircraft 100 occupies the centre of the “ROSE” 101 represented by a graduated circle. The shape and the colour of the intruders differ according to their associated degree of danger and according to the TCAS operating mode.
As examples, the aircraft 102 is close, at a relative altitude of 1100 feet under the aircraft 100, the relative altitude being symbolized by the indication “−11”. This aircraft is climbing, symbolized by an up-pointing arrow in FIG. 1. It is represented by a solid diamond coloured white or cyan representing a threat in PT (Proximate Traffic) mode. According to the aeronautical conventions, when the diamond is solid, the threat is of PT type, if it is empty, then the threat is of OT type, meaning “Other Traffic”.
The aircraft 103 is a threat in RA “Resolution Advisory” mode. It is situated at a relative altitude of 100 feet under the aircraft 100 and climbing. The colour of the square that represents it is red.
The aircraft 104 is an intruder in TA “Traffic Advisory” mode, it is 900 feet above the aircraft 100 and descending. The colour of the circle that represents it is amber.
As can be seen, the interpretation of the information by the pilot is far from immediate, which can prove particularly dangerous in cases of imminent risk of collision.
The new Synthetic Vision Systems SVS currently give the pilots a synthetic representation of the outside world and therefore, a better awareness of the surrounding dangers such as collisions with the ground without loss of control, commonly called CFIT (Controlled Flight Into Terrain). These SVS systems can currently display in 3D a synthetic terrain and the natural or artificial obstacles (buildings, etc.). An improvement on the presentation of the information supplied by the TCAS has been proposed in the Honeywell patent application entitled “Perspective View Conformal Traffic Target Display”, published under the international number WO2007/002917A1. FIG. 2 shows an example of representation of the intruders on a screen 200 of PFD (Primary Flight Display) type according to the provisions of this patent application. The intruders are presented in 3D in a conformal manner, that is, positioned in their real placement in the landscape. Additional information is associated with the intruders to assist the pilot in locating their position, above or below a reference altitude and their degree of separation obtained through a variation of the size of the symbols. FIG. 2 shows, in a 3D conformal synthetic view of the terrain 201, the air traffic. This view also includes a representation 210 of the PFD information. Intruders are presented in the sector in front of the aeroplane. The intruders 204 and 205 are represented by squares that are larger or smaller depending on their relative distance to the aeroplane. Other symbols are added to assist the pilot in interpreting the relative altitude of the intruder relative to the aeroplane. Thus, the symbols 202 and 203 representative of the vertical masts give the position and the height of the intruders above the ground. This presentation is well suited to airliners which fly relatively at high altitude.
Although the new SVS systems give the pilot a better understanding of the situation of the intruders, in particular their type, their positioning, their behaviour, their performance, and so on, these new systems are inadequate for carrying out missions at low altitude. In practice, intruders are very rare for airliners flying on instruments, which follow pre-established flight plans in strict air corridors and are controlled from the ground with radars by air traffic organizations. However, helicopters or small aeroplanes can fly in large numbers at low altitude, for example, to assist in a rescue of a large number of victims, in the context of a “red” plan or in the context of civil accident prevention missions. These aircraft can arrive from the front on approaching, but also from the rear, be situated just below or just above the aircraft. In this case, the flight is essentially a visual flight, with no established flight plan and/or outside conventional radar coverage. The visibility conditions can be degraded if flying at night, if flying towards the sun, in the presence of smoke for fire missions, and so on. The aircraft also have more dynamic and more varied trajectories (turns, climbs, descents, etc.) than those of airliners.
Given these conditions, it is particularly important for the pilot to clearly understand where the intruders are in relation to himself and mainly their performance characteristics such as approach speed, degree of dangerousness, etc. Among intruders, there are:                Those that arrive from the front on approach, but cannot be seen or are seen too late;        Those that arrive from the rear on approach and that cannot be seen visually;        
Those that are just below and/or above and that are invisible to the pilot. This last configuration has already led to a large number of fatal accidents with the pilot choosing to climb and/or descend.